fix(vessels): pin the idle-render contract and move the entry to a fragment

- The trailing visibility frame uses the layer's injectable runtime timer and
  is cancelled by resetState as well as disable and destroy.
- An empty-feed regular pass records the camera pose, so later in-cadence
  frames do not keep scheduling trailing frames.
- New renderGovernor.test.mjs: a parked camera schedules nothing, a stopped
  camera gets exactly one trailing frame and no self-wake, a forced pass
  requests its frame, destroy cancels the timer.
- The CHANGELOG entry moves to changelog.d/871-vessels-idle-render.md; docs
  say the hold also covers vessels dimmed under a live target.

changelog.d/871-vessels-idle-render.md

@@ -0,0 +1,7 @@
+- Let the scene idle with AIS vessels enabled. Vessels no longer hold the
+  continuous render loop; they request frames for new data and selection, one
+  trailing frame after the camera stops, and hold the loop only while focus
+  emphasis animates or a vessel stays dimmed under a tracked target. A parked
+  vessels-only scene went from continuous rendering to 0 frames in 5 s. While
+  anything is tracked, the vessel focus pass skips hidden sprites at rest:
+  about 4 ms to 1.2 ms per pass over 12,000 vessels (waar19, #871).